The study was commissioned by the National Institute of Justice and conducted by Chilton Research Services of Drexel Hill, Pennsylvania. Researchers conducted a phone survey of more than 2,500 adults and asked them several questions about their guns, including where and how they got them. However, just 251 people answered questions about their firearms.

They found 60 percent of respondents said their guns were bought from either a gun store, pawn shop or some other store. Assuming those stores were all federally licensed dealers those purchases were subject to a background check. The remaining 40 percent said their guns came from gun shows, a family member, a friend, through the mail or some other means. Researchers estimated that added up to two million sales and transfers per year that were "off the book."

However, some opponents of stricter gun control laws say this number is inaccurate. John Lott, economist, gun rights supporter and the author of the book "More Guns, Less Crime," wrote in the National Review in January that the research contains bias. He said if the guns inherited from family members and friends were originally bought from licensed dealers then the percentage of gun sales or transfers without a background check fell to around 11.5 percent, not 40 percent.

However, the survey does not specify if the gun owners surveyed were themselves eligible to pass a background check for the firearm they received from a family member. Researchers say they spoke with adults who were fluent in English or Spanish and were non-institutionalized.

Transfers of guns between family members are not unusual. Rep. Michael Paymar, DFL-St. Paul, who authored a bill requiring background checks for nearly all private gun sales or transfers, exempted transactions between relatives.

But a competing House proposal does not include extended background checks. Rep. Debra Hilstrom, a Brooklyn Center Democrat, is expected this week to unveil a bill that focuses more on tougher penalties for those who possess guns illegally. The bill is backed by Minnesota sheriffs.

Universal background checks get wide support in local and nationwide polls, even when among members of the National Rifle Association.